1. Field of the Invention
The present invention relates to an image forming apparatus that conveys a sheet material stored in a sheet tray to a transfer position and transfers an image onto the sheet material.
2. Description of the Related Art
In conventional image forming apparatuses, in conveying a sheet material such as a cut sheet stored in a sheet tray to a transfer position on a conveying path, the leading end of the sheet material is made to abut on a pair of rollers (registration rollers) disposed upstream of the transfer position so that the sheet material is corrected for skewing with respect to a direction in which the sheet material moves, and carries out timing adjustment by causing the sheet material to wait for a predetermined period of time so that the leading end of the sheet material and the leading end of an image can be aligned with each other.
Further, in the conventional image forming apparatuses, in transferring a color image onto a sheet material, a toner image on a photosensitive member is directly transferred onto the sheet material.
However, the conventional image forming apparatuses that directly transfer a toner image on the photosensitive member onto a sheet material have the problem that the transfer efficiency depends on physical characteristics of a sheet material being conveyed. To solve this problem, an image forming apparatus has been proposed which transfers a toner image on a photosensitive member onto an intermediate transfer member and transfers the toner image on the intermediate transfer member onto the sheet material.
FIGS. 8A and 8B schematically show the constructions of the above described image forming apparatuses.
FIG. 8A is a view showing an example of the image forming apparatus that directly transfers a toner image on the photosensitive member onto a sheet material, and FIG. 8B is a view showing an example of the image forming apparatus that transfers a toner image on the photosensitive member onto the intermediate transfer member and then transfers the toner image on the intermediate transfer member onto the sheet material.
In FIGS. 8A and 8B, reference numeral 101 denotes a sheet tray; 102, a pair of sheet feed rollers; 103, a pair of drawing rollers; 104, a pair of registration rollers; 105, a photosensitive drum for forming a black (K) toner image; 106, a photosensitive drum for forming a cyan (C) toner image; 107, a photosensitive drum for forming a magenta (M) toner image; 108, a photosensitive drum for forming an yellow (Y) toner image; 109, a fixing device; 120, a pick-up roller; 121, a transfer roller; 122, an intermediate transfer member; 123, a conveying belt; and P, a sheet material.
To form a color image on the sheet material P, image forming units that form respective ones of Y, M, C, and K toner images are arranged in a direction in which the sheet material P is conveyed.
Referring to FIG. 8A, a description will now be given of a process in which a color image is formed on the sheet material P.
First, the sheet material P set on the sheet tray 101 is conveyed to the pair of registration rollers 104 by the pick-up roller 120, the pair of sheet feed rollers 102, and the pair of drawing rollers 103, where the leading end of the sheet material P abuts on the pair of registration rollers 104. Then, upon the lapse of a waiting time, the pair of registration rollers 104 start rotating.
The exposure of an image on the photosensitive drum 105 for forming a K toner image is started in such timing that the leading end of the sheet material P and the leading end of the image are aligned with each other. Thereafter, images are successively exposed on the photosensitive drum 106 for forming a C toner image, the photosensitive drum 107 for forming an M toner image, and the photosensitive drum 108 for forming a Y toner image, and the images are superposed one over the other on the sheet material P conveyed by the sheet material conveying belt 123 to form a full-color image.
After the toner images are transferred onto the sheet material P, the toner images are fixed on the sheet material P by the fixing device 109 and then discharged.
Referring next to FIG. 8B, a description will be given of a process in which a color image is formed on the sheet material P.
Similarly to the image forming apparatus in FIG. 8A, the image forming apparatus in FIG. 8B is comprised of the photosensitive drums (105 to 108) for forming Y, M, C, and K toner images, the pick-up roller 120, the pair of sheet feed rollers 102, the pair of drawing rollers 103, and the pair of registration rollers 104, which are used for conveying the sheet material P set on the sheet tray 101.
Toner images transferred onto the intermediate transfer member 122 are transferred onto the sheet material P via the transfer roller 121 and then fixed by the fixing device 109.
As shown in FIG. 8B, in the image forming apparatus employing the intermediate transfer member 122, there is a long distance from the photosensitive drum 108 to the transfer roller 121, and hence a period of time in which a toner image on the photosensitive drum 108 is conveyed to the transfer roller 121 via the intermediate transfer member 122 can be longer than a period of time in which the leading end of the sheet material P is fed from the sheet tray 101 to the transfer roller 121. For this reason, the image forming apparatus of this type needs to start image exposure before starting the conveyance of the sheet material P.
However, in the case where image exposure is started before the conveyance of the sheet material P is started, if there is a delay in conveyance of the sheet material P, the problem that the leading end of the sheet material P and the leading end of the toner image are not aligned with each other, i.e. misalignment of leading ends occurs.
Therefore, it is necessary to provide control to compensate for a delay in conveyance of the sheet material P.
Conventionally, the following ways of conveyance control have been envisaged so as to compensate for a delay in conveyance of the sheet material P: (1) a period of time in which the sheet material P is conveyed over a predetermined section is measured and the measured period of time is compared with a reference period of time in which the sheet material P passes through the predetermined section, and if the period of time in which the sheet material P is conveyed is longer than the reference period of time, the conveying speed is increased; and (2) a period of time in which the sheet material P is conveyed over a predetermined section is measured and the measured period of time is compared with a reference period of time in which the sheet material P passes through the predetermined section, and image exposure start timing is delayed in accordance with a delay in conveyance, which is obtained by the comparison.
However, according to the above way of control (1), a motor for conveying the sheet material P needs to be rotated at a higher speed than the normal conveying speed, and hence it is necessary to use a motor whose driving speed can be varied in a wide range. This increases the cost of the image forming apparatus.
The above way of control (2) is useful for sheet material conveyance control by the image forming apparatus that directly transfers a toner image onto the sheet material P.
However, if the way of control (2) is applied to the image forming apparatus employing the intermediate transfer member, it is impossible to prevent leading end misalignment by compensating for a delay in conveyance of the sheet material P because image exposure has already been started before the start of conveyance.